1. Technical Field
The present invention relates to a fixing device, a heater control method, and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with a heated, endless fuser belt, a heater control method for use in the fixing device, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features, that incorporates the fixing device.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of these features, an image is formed by attracting developer or toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process may be followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium with heat and pressure.
In general, a fixing device includes a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip. As a recording medium bearing a toner image thereupon enters the fixing nip, the fuser member heats the recording medium to fuse and melt the toner particles, while the pressure member presses the recording medium against the fuser member to fix the molten toner onto the recording medium.
Modern fixing processes used in power-saving printers are designed with low power consumption. For example, some fixing devices save power consumed in heating a fuser member by activating an electronic heater only where necessary to execute printing, that is, starting power supply to the heater only after receiving a print request, and otherwise leaving the heater deactivated.
One important aspect of such a fixing device is the capability to reduce a period of warm-up time required for to heat the fuser member to a setpoint temperature upon activation. For power-saving applications, a long warm-up time is usually undesirable, since it means a prolonged period of inertia during which the printer cannot perform printing, resulting in a correspondingly long period of first-print time between reception and completion of a first print request submitted upon power-on of the printer.
In an effort to provide a fixing process with shorter warm-up and first-print times, a belt-based fixing device has been proposed based on an endless fuser belt that exhibits a lower heat capacity compared to that of a cylindrical roller. This fixing device includes a pair of opposed rotary members, one being a fuser belt looped into a generally cylindrical configuration, and the other being a generally cylindrical, rotatable pressure member, pressed against each other to form a fixing nip therebetween, through which a recording medium is conveyed under heat and pressure as the rotary fixing members rotate together.
The fuser belt assembly has a radiant, halogen heater disposed inside the belt loop for directly radiating heat. Also, the fuser belt is provided with a black coating material deposited on the belt inner circumferential surface for promoting generation of radiant heat in the belt. According to this method, direct radiation with the halogen heater, which accelerate heating of the belt, in conjunction with provision of the black coating, which increases thermal efficiency in radiant heating, results in a short warm-up time, such as, for example, of 10 seconds or even shorter in the fixing device.
To date, several fixing processes are known provided with a capability to detect an abnormal condition, that is, a possible anomaly of a heating control system in which a thermometer fails to correctly measure an operational temperature of a fuser member, and/or a heater and its power supply do not function properly to heat a fuser member to a desired setpoint temperature.
For example, a belt-based fixing device is known employing an endless fuser belt equipped with a halogen lamp as well as a thermometer, which incorporates an overheat detection capability to detect an abnormal condition where an operational temperature detected by the thermometer exceeds a maximum threshold temperature, indicating overheating of the fuser belt.
Another fixing device is known that employs a cylindrical fuser roller, instead of a fuser belt, equipped with a heater as well as a thermometer, which incorporates an underheat detection capability to detect an abnormal condition where an operational temperature detected by the thermometer does not reach a minimum threshold temperature upon lapse of a warm-up time limit since activation of the heater, indicating underheating of the fuser belt or misdetection of the thermometer.
Upon detecting an abnormal condition, these fixing devices may deactivate the heater and stop conveyance of recording media, thereby suspending execution of a current print job, while notifying a user of printing failure by displaying an alert message on a control panel which prompts a user to call a customer service number.
The inventor has recognized difficulties associated with anomaly detection in a belt-based fixing device, which arise from aging or other temporal changes in operational conditions of the fuser belt assembly, causing variations in the speed or efficiency with which the heater heats the fuser belt to a desired, setpoint temperature.
Specifically, the heating speed of the fuser belt may decrease over time because of various factors caused by an extended period of use of the fixing device. For example, one such factor is aging of the belt heater, which leads to a natural reduction in heating performance. Another such factor is gradual deposition of foreign matter on the belt heater (e.g., grease or lubricant agent migrating from the belt inner circumferential surface to the heater), which can adversely affect electromagnetic radiation, in particular, where the heater is configured as a radiant, halogen heater.
Still another factor occurs where the fixing device uses a reflector or reflective surface that directs electromagnetic radiation from the heater to the fuser belt for increasing thermal efficiency. As is the case with the belt heater, gradual deposition of foreign matter on the reflector adversely affects electromagnetic reflection, resulting in a concomitant reduction in the radiant heat generated in the fuser belt.
Yet still another factor occurs where the fuser belt is provided with a black coating material on its inner circumferential surface for increasing thermal efficiency. In this case, abrasion on the belt surface due to sliding against an adjoining element, such as a fuser pad, results in thinning of the black coating and smoothening of the belt surface, both of which adversely affect electromagnetic absorption in the fuser belt, resulting in a concomitant reduction in the radiant heat generated in the fuser belt.
Reduced thermal efficiency and decelerated heating of the fuser belt translates into a prolonged period of time required for the belt temperature to reach a setpoint temperature upon activation of the heater. Accordingly, as the fuser belt assembly ages or is more used, a warm-up time limit for anomaly detection, designed to work with the fixing device in its new, unaged state, becomes less effective, since prematurely expires before the belt temperature reaches the setpoint even where the heating control system is properly operating. If not corrected, premature expiration of the warm-up time limit causes the anomaly detector to mistakenly detect an abnormal condition that actually does not exist.
To counteract the problem, one possible measure is to establish a sufficiently long warm-up time limit that does not expire prematurely insofar as its heating control system properly operates regardless of whether the fixing device is in its unaged state or aged state.
Unfortunately, however, such a measure is not practical or even effective, because setting a long warm-up time limit in a newly installed fixing device would in turn hinder correct and immediate anomaly detection in response to occurrence of an abnormal condition, leading to various adverse consequences.
For example, failure to timely detect an anomaly may cause a delay in submitting a notification of printing failure to allow a user to reallocate the failed print job to an alternative printer, resulting in a significant delay in providing a printed output, which detracts from user convenience and utility of the printer. Also, delayed anomaly detection can cause a delay in repair or servicing the fixing device for correcting an abnormal condition detected, resulting in a prolonged downtime of the printer. Further, delayed anomaly detection can damage the fuser belt where the anomaly detector ignores a thermometer failure to correctly detect a fuser belt temperature, in which case the heater does not stop heating even where the belt temperature exceeds a setpoint temperature, resulting in overheat and concomitant thermal damage to the fuser belt.